The invention concerns a drilling apparatus for enlarging a borehole in the ground. The drilling apparatus includes a drilling head which has a drilling bit, a percussion piston which is driven by a drive medium and impacts against the drilling bit, and a hollow drill string to which the drilling head is fixed and through the cavity of which the drive medium is passed to the percussion piston.
The invention further concerns a method of enlarging an existing borehole, in which a drilling head is fixed to a hollow drill string introduced into the borehole and is drawn through the borehole, wherein a percussion piston which is driven by a drive medium which is fed through the hollow drill string impacts on a drilling bit of the drilling head during the drilling operation.
A drilling apparatus of that kind which operates in accordance with the specified method is described in U.S. Pat. No 5,791,419. The drilling apparatus described therein serves for drilling out an existing pipe in the ground and for replacing that pipe by a fresh pipe. The hollow drill string is firstly pushed through the pipe which is to be drilled out. When it issues at the end of the pipe to be drilled out the drilling head is fixed to the hollow drill string. The drilling crown or annular drilling arrangement, that is to say the surface of the drilling head which is fitted with the cutting edges or teeth and which provides for removal of the drilling waste material, faces in that situation towards the drill string. A percussion piston within the drill head which is of an annular configuration and surrounds the drill string can apply hammer blows to the rear of a drill bit which carries the annular drilling arrangement. The drill bit is axially movably fixed to the drilling head by way of a damping element.
The drive medium for actuation of the percussion pistonxe2x80x94usually compressed airxe2x80x94is passed through the interior of the hollow drill string to the drilling head and to the percussion piston. After flowing through the passages in the drilling head and driving the percussion piston the drive medium flows through a passage to the annular drilling arrangement of the drilling bit. From here it can flow in the pipe to be renovated in parallel relationship with the drill string back again towards the opening of the pipe to be drilled out.
This return flow of the drive medium which possibly blows the drilling waste material out of the borehole is effected in the annular space around the drill string which is guided at a large radial spacing relative to the pipe to be renewed. In order to provide for drilling out the pipe in substantially parallel relationship with its axis that drilling head must have a guide body and centering body. If the drill string is guided through a blocked region or a water-filled region of the pipe to be renewed, there is the danger that the return flow of the drive medium is impeded or interrupted.
One object of the invention is to develop a method and an apparatus for enlargement drilling of the specified kind, in such a way as to ensure that the drive medium is blown out without disturbance.
In regard to one embodiment of the drilling apparatus that object is attained in that the drill string is surrounded by a hollow outer string which encloses an annular space which surrounds the drill string and through which the drive medium is passed out of the borehole.
In accordance with the apparatus of the invention according to one embodiment, an annular second passage within a string in the form of a double string affords a passage which is separate from the surroundings in the borehole and through which the drive medium which is guided into the borehole can flow after driving the percussion piston in order to be passed out of the borehole. It is possible in that way to prevent materials from the ground around the string from being flushed out of the borehole together with the drive medium. For example if the borehole extends below the ground water table the amount of water which is blown out of the borehole jointly with the drive medium is reduced by the outer drill string through which the drive medium flows back.
The method according to one embodiment of the invention is preferably used for enlarging a pilot bore which was produced in a directional drilling process. In the case of directional drilling a support pipe is generally drawn into the borehole behind a steerable directional drilling head. At the same time a support medium is supplied, which usually comprises bentonite mixed with water. In that case the support pipe which is drawn in during the production of a pilot bore with the directional drilling head is preferably used as an outer string of the enlargement drilling apparatus. That support pipe is surrounded flush by the borehole. The aqueous bentonite which surrounds the support pipe is of a jelly-like consistency which permits axial movement of the support pipe with a very low level of frictional resistance. It will be noted however that it closes an annular space outside the string, through which the drive medium could flow out. The outer string of the drilling apparatus according to one embodiment of the invention ensures that the support medium is not undesirably blown out of the borehole by the drive medium for the percussion hammer. The drive medium can flow freely jointly with the drilling waste material through the annular space between the inner drill string and the outer string towards the end of the borehole.
As, unlike the situation with conventional drilling processes, the drill string applies a pulling force to the drilling head in the enlargement drilling procedure, the drill string preferably projects through the drilling head and is connected to the rear thereof. The percussion piston is preferably of an annular configuration, surrounds the drill string and impacts against the rear of the drill bit of the drilling head.
The annular drilling arrangement, that is to say the surface which provides for removal of the ground, is arranged at the side of the drilling head, that is remote from the end of the drill string.
The drive medium is thus guided through the inner drill string which extends through the drilling head. Screwed on the end of the inner drill string is a closure cover which seals off the interior of the drill string and applies the pulling force from the drill string to the drilling head. The drilling head is of a substantially cylindrical shape. A sleeve-shaped piston is displaceably guided in the drilling head. Passages for the flow of the drive medium into the annular region in which the percussion piston is accommodated also extend in the drilling head and in the closure cover. The drive medium flows through preferably pneumatic control means which cause the piston movement and which have been known for years in connection with pneumatic deep hole drilling hammers. The impact surface of the drilling bit is disposed at the end of the flow path for the drive medium and at the end of the space for movement of the percussion piston. A passage for the flow of the drive medium also passes through the drilling bit and opens in the region of the annular drilling arrangement. In that way the drive medium is firstly passed through the inner drill string and then through the drilling head to the surface of the annular drilling arrangement, which provides for the removal of material. Disposed in front of the annular drilling arrangement in the pulling direction is at least one entry opening in the outer string, through which the drive medium which has been passed into the bottom of the enlargement bore can pass. The above-mentioned drive medium will thus flow by virtue of the increased pressure jointly with the removed material (drilling waste material) into the annular space between the outer string and the inner drill string and is transported through that annular space out of the borehole.
Preferably a rotary movement is transmitted to the annular drilling arrangement by way of the drill string. Usually, a string which is composed of individual string segments is used when producing a borehole in the ground which is at least 10 m and often over 100 m in length. In that case the inner drill string and the outer string are preferably screwed together by way of interengaging female and male screwthreads. As the vibrations caused by the percussion piston generally loosen a screw connection of that kind, a rotary drive in the screwing direction is utilized in order to prevent unwanted unscrewing and loosening of the drill string. Obviously and in particular the rotary movement which is transmitted by way of the drill string increases the material-removal capacity of the annular drilling arrangement.
Particularly preferably, the inner string and the outer string are fixed to a common coupling element outside the borehole. On the one hand the rotary movement is transmitted by way of the coupling element jointly to the inner drill string and to the outer string. On the other hand the drive medium is introduced by way of the coupling element into the interior of the inner drill string. The coupling element also has outlet openings for the drive medium which flows out of the borehole. The coupling element is preferably arranged on a forward drive machine having a linear drive which transmits the pulling force to the coupling element and by way of the coupling element to the inner drill string and the outer string.
The rotary force can be transmitted to the annular drilling arrangement either solely by the inner string or solely by the outer string or by both strings, at the same time.
As in the state of the art, the drill bit with the annular drilling arrangement is damped elastically with respect to the drill string. That reduces transmission of the percussion forces of the percussion piston from the drilling bit to the drill string. Thus the substantial part of the energy of the percussion piston for material removal is applied by way of the drilling bit to the end face of the annular drilling arrangement. Only a small proportion of that energy is transmitted to the drill string so that the latter is not overloaded.
Preferably a connecting element to which a support pipe is fixed is mounted at the end of the drilling head which is the rear end in the pulling direction. As already mentioned the drilling apparatus according to one embodiment of the invention is preferably used for enlargement drilling of pilot bores produced by a directional drilling process. The support pipe of the first enlargement bore which in many cases already forms the definitive service pipe which is drawn into the bore (for example for carrying fresh water, cables etc) prevents the bore produced from collapsing behind the drilling head. The support pipe is preferably drawn linearly through the borehole without a rotary movement and for that reason is decoupled from the rotary movement of the drill string.
The end of the support pipe which is drawn through the borehole is preferably closed by way of a head plate which is a component part of the connecting element. Preferably the head plate has at least one outlet passage for a support medium which is introduced through the support pipe into the borehole near the drilling head. As already mentioned, preferably bentonite mixed with water is used as the support medium, being of a gel-like consistency and involving a low level of frictional resistance so that the operation of drawing in the support pipe is made easier.
The surface of the head plate of the support pipe, which faces towards the drill string, preferably has at least in the outer region of its periphery a surface which extends inclinedly with respect to the radial direction of the drill string and which extends in a pointed configuration in the pulling direction. The preferably conical surface of the head plate provides for displacement of the drilling mud into the peripheral regions of the support pipe.
As mentioned above the novel drilling apparatus can carry out a method according to one embodiment of the invention for the enlargement of an existing borehole, in which a drilling head is fixed to a hollow drill string introduced into the borehole and is drawn through the borehole, wherein a percussion piston which is driven by a drive medium which is fed through the hollow drill string impacts against a drilling bit of the drilling head during the drilling procedure.
In accordance with one embodiment of the invention and to attain the above-specified object an outer string surrounding the drill string is drawn jointly therewith through the borehole, wherein enclosed between the outer string and the drill string is an annular space through which the drive medium is passed out of the borehole.
The method according to one embodiment of the invention is particularly preferably used for enlarging pilot bores which were produced by a directional drilling process. In the directional drilling process a steerable directional drilling head is driven through the ground. The directional drilling head is so designed that it produced a borehole whose longitudinal axis is curved in a given direction. The direction of the curvature can be varied by rotating the directional drilling head. If drilling is to be effected straight, the directional drilling head is rotated continuously about its longitudinal axis so that the curvature of the borehole is uniformly distributed in all directions and is thus cancelled out. In accordance with the state of the art a directional drilling head is pushed with a directional drill string through the borehole, wherein a support medium is introduced into the borehole, which bears flush against the directional drill string. After completion of the pilot bore that directional drill string forms the outer string for the drilling apparatus according to the invention.
In accordance with a hitherto unpublished European application to the present applicant with the application No 01 201 167.2, in directional drilling, with a percussion drill string introduced in the directional drill string, percussion forces can be transmitted to the directional drilling head. It will be appreciated that this percussion string is to be driven out of the outer string before introduction of the inner drill string. The percussion string in accordance with the above-identified European patent application comprises string segments which bear against each other and which are not screwed to each other. The string segments can be driven out for example through one end of the outer string, by a compressed air source being connected to the other end of the outer string.
As mentioned, a support pipe is preferably fixed to the side of the drilling head, which is opposite to the drill string, the support pipe being drawn jointly with the drilling head through the enlarged borehole. The above-mentioned support medium can be passed into the borehole through that support pipe.
As mentioned the drilling performance is preferably increased by using a rotary drive coupled to the drilling head. In that case the support pipe is preferably decoupled from the rotary movement of the drill string.